Fixed race condition in PollingDeviceDiscovery. (#145506)

There are two issues in the previous implementation:
1. `_populateDevices` will return the devices from `deviceNotifier` if it had been initialized, assuming that once it's initialized, it has been properly populated. That assumption is not true because calling getters like `onAdded` would initialize `deviceNotifier` without populating it.
2. `deviceNotifier` instance might be replaced in some cases, causing `onAdded` subscribers to lose any future updates.

To fix (1), this commit added the `isPopulated` field in `deviceNotifier` as a more accurate flag to determine if we need to populate it.

To fix (2), this commit made `deviceNotifier` a final member in `PolingDeviceDiscovery`.

packages/flutter_tools/lib/src/base/utils.dart

@@ -97,9 +97,9 @@ String getSizeAsMB(int bytesLength) {
 /// removed, and calculate a diff of changes when a new list of items is
 /// available.
 class ItemListNotifier<T> {
-  ItemListNotifier(): _items = <T>{};
+  ItemListNotifier(): _items = <T>{}, _isPopulated = false;
 
-  ItemListNotifier.from(List<T> items) : _items = Set<T>.of(items);
+  ItemListNotifier.from(List<T> items) : _items = Set<T>.of(items), _isPopulated = true;
 
   Set<T> _items;
 
@@ -111,6 +111,11 @@ class ItemListNotifier<T> {
 
   List<T> get items => _items.toList();
 
+  bool _isPopulated;
+
+  /// Returns whether the list has been populated.
+  bool get isPopulated => _isPopulated;
+
   void updateWithNewList(List<T> updatedList) {
     final Set<T> updatedSet = Set<T>.of(updatedList);
 
@@ -118,9 +123,10 @@ class ItemListNotifier<T> {
     final Set<T> removedItems = _items.difference(updatedSet);
 
     _items = updatedSet;
+    _isPopulated = true;
 
-    addedItems.forEach(_addedController.add);
     removedItems.forEach(_removedController.add);
+    addedItems.forEach(_addedController.add);
   }
 
   void removeItem(T item) {

packages/flutter_tools/lib/src/device.dart

@@ -480,18 +480,15 @@ abstract class PollingDeviceDiscovery extends DeviceDiscovery {
 
   @protected
   @visibleForTesting
-  ItemListNotifier<Device>? deviceNotifier;
+  final ItemListNotifier<Device> deviceNotifier = ItemListNotifier<Device>();
 
   Timer? _timer;
 
   Future<List<Device>> pollingGetDevices({Duration? timeout});
 
   void startPolling() {
-    if (_timer == null) {
-      deviceNotifier ??= ItemListNotifier<Device>();
-      // Make initial population the default, fast polling timeout.
-      _timer = _initTimer(null, initialCall: true);
-    }
+    // Make initial population the default, fast polling timeout.
+    _timer ??= _initTimer(null, initialCall: true);
   }
 
   Timer _initTimer(Duration? pollingTimeout, {bool initialCall = false}) {
@@ -499,7 +496,7 @@ abstract class PollingDeviceDiscovery extends DeviceDiscovery {
     return Timer(initialCall ? Duration.zero : _pollingInterval, () async {
       try {
         final List<Device> devices = await pollingGetDevices(timeout: pollingTimeout);
-        deviceNotifier!.updateWithNewList(devices);
+        deviceNotifier.updateWithNewList(devices);
       } on TimeoutException {
         // Do nothing on a timeout.
       }
@@ -546,32 +543,28 @@ abstract class PollingDeviceDiscovery extends DeviceDiscovery {
     DeviceDiscoveryFilter? filter,
     bool resetCache = false,
   }) async {
-    if (deviceNotifier == null || resetCache) {
+    if (!deviceNotifier.isPopulated || resetCache) {
       final List<Device> devices = await pollingGetDevices(timeout: timeout);
       // If the cache was populated while the polling was ongoing, do not
       // overwrite the cache unless it's explicitly refreshing the cache.
-      if (resetCache) {
-        deviceNotifier = ItemListNotifier<Device>.from(devices);
-      } else {
-        deviceNotifier ??= ItemListNotifier<Device>.from(devices);
+      if (!deviceNotifier.isPopulated || resetCache) {
+        deviceNotifier.updateWithNewList(devices);
       }
     }
 
     // If a filter is provided, filter cache to only return devices matching.
     if (filter != null) {
-      return filter.filterDevices(deviceNotifier!.items);
+      return filter.filterDevices(deviceNotifier.items);
     }
-    return deviceNotifier!.items;
+    return deviceNotifier.items;
   }
 
   Stream<Device> get onAdded {
-    deviceNotifier ??= ItemListNotifier<Device>();
-    return deviceNotifier!.onAdded;
+    return deviceNotifier.onAdded;
   }
 
   Stream<Device> get onRemoved {
-    deviceNotifier ??= ItemListNotifier<Device>();
-    return deviceNotifier!.onRemoved;
+    return deviceNotifier.onRemoved;
   }
 
   void dispose() => stopPolling();

packages/flutter_tools/lib/src/ios/devices.dart

@@ -102,8 +102,6 @@ class IOSDevices extends PollingDeviceDiscovery {
       return;
     }
 
-    deviceNotifier ??= ItemListNotifier<Device>();
-
     // Start by populating all currently attached devices.
     _updateCachedDevices(await pollingGetDevices());
     _updateNotifierFromCache();
@@ -127,10 +125,8 @@ class IOSDevices extends PollingDeviceDiscovery {
 
   @visibleForTesting
   Future<void> onDeviceEvent(XCDeviceEventNotification event) async {
-    final ItemListNotifier<Device>? notifier = deviceNotifier;
-    if (notifier == null) {
-      return;
-    }
+    final ItemListNotifier<Device> notifier = deviceNotifier;
+
     Device? knownDevice;
     for (final Device device in notifier.items) {
       if (device.id == event.deviceIdentifier) {
@@ -186,10 +182,8 @@ class IOSDevices extends PollingDeviceDiscovery {
   /// Updates notifier with devices found in the cache that are determined
   /// to be connected.
   void _updateNotifierFromCache() {
-    final ItemListNotifier<Device>? notifier = deviceNotifier;
-    if (notifier == null) {
-      return;
-    }
+    final ItemListNotifier<Device> notifier = deviceNotifier;
+
     // Device is connected if it has either an observed usb or wifi connection
     // or it has not been observed but was found as connected in the cache.
     final List<Device> connectedDevices = _cachedPolledDevices.values.where((Device device) {

packages/flutter_tools/test/general.shard/base_utils_test.dart

@@ -36,5 +36,27 @@ void main() {
       expect(removedItems.first, 'aaa');
       expect(removedItems[1], 'bbb');
     });
+
+    test('becomes populated when item is added', () async {
+      final ItemListNotifier<String> list = ItemListNotifier<String>();
+      expect(list.isPopulated, false);
+      expect(list.items, isEmpty);
+
+      // Becomes populated when a new list is added.
+      list.updateWithNewList(<String>['a']);
+      expect(list.isPopulated, true);
+      expect(list.items, <String>['a']);
+
+      // Remain populated even when the last item is removed.
+      list.removeItem('a');
+      expect(list.isPopulated, true);
+      expect(list.items, isEmpty);
+    });
+
+    test('is populated by default if initialized with list of items', () async {
+      final ItemListNotifier<String> list = ItemListNotifier<String>.from(<String>['a']);
+      expect(list.isPopulated, true);
+      expect(list.items, <String>['a']);
+    });
   });
 }

packages/flutter_tools/test/general.shard/device_test.dart

@@ -1089,6 +1089,47 @@ void main() {
       );
     });
   });
+
+  group('PollingDeviceDiscovery', () {
+    final FakeDevice device1 = FakeDevice('Nexus 5', '0553790d0a4e726f');
+
+    testWithoutContext('initial call to devices returns the correct list', () async {
+      final List<Device> deviceList = <Device>[device1];
+      final TestPollingDeviceDiscovery testDeviceDiscovery = TestPollingDeviceDiscovery(deviceList);
+
+      // Call `onAdded` to make sure that calling `onAdded` does not affect the
+      // result of `devices()`.
+      final List<Device> addedDevice = <Device>[];
+      final List<Device> removedDevice = <Device>[];
+      testDeviceDiscovery.onAdded.listen(addedDevice.add);
+      testDeviceDiscovery.onRemoved.listen(removedDevice.add);
+
+      final List<Device> devices = await testDeviceDiscovery.devices();
+      expect(devices.length, 1);
+      expect(devices.first.id, device1.id);
+    });
+
+    testWithoutContext('call to devices triggers onAdded', () async {
+      final List<Device> deviceList = <Device>[device1];
+      final TestPollingDeviceDiscovery testDeviceDiscovery = TestPollingDeviceDiscovery(deviceList);
+
+      // Call `onAdded` to make sure that calling `onAdded` does not affect the
+      // result of `devices()`.
+      final List<Device> addedDevice = <Device>[];
+      final List<Device> removedDevice = <Device>[];
+      testDeviceDiscovery.onAdded.listen(addedDevice.add);
+      testDeviceDiscovery.onRemoved.listen(removedDevice.add);
+
+      final List<Device> devices = await testDeviceDiscovery.devices();
+      expect(devices.length, 1);
+      expect(devices.first.id, device1.id);
+
+      await pumpEventQueue();
+
+      expect(addedDevice.length, 1);
+      expect(addedDevice.first.id, device1.id);
+    });
+  });
 }
 
 class TestDeviceManager extends DeviceManager {
@@ -1203,3 +1244,24 @@ class ThrowingPollingDeviceDiscovery extends PollingDeviceDiscovery {
   @override
   List<String> get wellKnownIds => <String>[];
 }
+
+
+class TestPollingDeviceDiscovery extends PollingDeviceDiscovery {
+  TestPollingDeviceDiscovery(this._devices) : super('test');
+
+  final List<Device> _devices;
+
+  @override
+  Future<List<Device>> pollingGetDevices({ Duration? timeout }) async {
+    return _devices;
+  }
+
+  @override
+  bool get supportsPlatform => true;
+
+  @override
+  bool get canListAnything => true;
+
+  @override
+  List<String> get wellKnownIds => <String>[];
+}

packages/flutter_tools/test/general.shard/ios/devices_test.dart

@@ -659,7 +659,7 @@ void main() {
       await iosDevices.startPolling();
       expect(xcdevice.getAvailableIOSDevicesCount, 1);
 
-      expect(iosDevices.deviceNotifier!.items, isEmpty);
+      expect(iosDevices.deviceNotifier.items, isEmpty);
       expect(xcdevice.deviceEventController.hasListener, isTrue);
 
       xcdevice.deviceEventController.add(
@@ -670,9 +670,9 @@ void main() {
         ),
       );
       await added.future;
-      expect(iosDevices.deviceNotifier!.items.length, 2);
-      expect(iosDevices.deviceNotifier!.items, contains(device1));
-      expect(iosDevices.deviceNotifier!.items, contains(device2));
+      expect(iosDevices.deviceNotifier.items.length, 2);
+      expect(iosDevices.deviceNotifier.items, contains(device1));
+      expect(iosDevices.deviceNotifier.items, contains(device2));
       expect(iosDevices.eventsReceived, 1);
 
       iosDevices.resetEventCompleter();
@@ -684,9 +684,9 @@ void main() {
         ),
       );
       await iosDevices.receivedEvent.future;
-      expect(iosDevices.deviceNotifier!.items.length, 2);
-      expect(iosDevices.deviceNotifier!.items, contains(device1));
-      expect(iosDevices.deviceNotifier!.items, contains(device2));
+      expect(iosDevices.deviceNotifier.items.length, 2);
+      expect(iosDevices.deviceNotifier.items, contains(device1));
+      expect(iosDevices.deviceNotifier.items, contains(device2));
       expect(iosDevices.eventsReceived, 2);
 
       iosDevices.resetEventCompleter();
@@ -698,9 +698,9 @@ void main() {
         ),
       );
       await iosDevices.receivedEvent.future;
-      expect(iosDevices.deviceNotifier!.items.length, 2);
-      expect(iosDevices.deviceNotifier!.items, contains(device1));
-      expect(iosDevices.deviceNotifier!.items, contains(device2));
+      expect(iosDevices.deviceNotifier.items.length, 2);
+      expect(iosDevices.deviceNotifier.items, contains(device1));
+      expect(iosDevices.deviceNotifier.items, contains(device2));
       expect(iosDevices.eventsReceived, 3);
 
       xcdevice.deviceEventController.add(
@@ -711,7 +711,7 @@ void main() {
         ),
       );
       await removed.future;
-      expect(iosDevices.deviceNotifier!.items, <Device>[device2]);
+      expect(iosDevices.deviceNotifier.items, <Device>[device2]);
       expect(iosDevices.eventsReceived, 4);
 
       iosDevices.resetEventCompleter();
@@ -777,7 +777,7 @@ void main() {
       xcdevice.devices.add(<IOSDevice>[]);
 
       await iosDevices.startPolling();
-      expect(iosDevices.deviceNotifier!.items, isEmpty);
+      expect(iosDevices.deviceNotifier.items, isEmpty);
       expect(xcdevice.deviceEventController.hasListener, isTrue);
 
       iosDevices.dispose();

packages/flutter_tools/test/general.shard/proxied_devices/proxied_devices_test.dart

@@ -539,11 +539,11 @@ void main() {
 
       proxiedDevices.startPolling();
 
-      final ItemListNotifier<Device>? deviceNotifier = proxiedDevices.deviceNotifier;
+      final ItemListNotifier<Device> deviceNotifier = proxiedDevices.deviceNotifier;
       expect(deviceNotifier, isNotNull);
 
       final List<Device> devicesAdded = <Device>[];
-      deviceNotifier!.onAdded.listen((Device device) {
+      deviceNotifier.onAdded.listen((Device device) {
         devicesAdded.add(device);
       });